1. Technical Field of Invention
This invention relates generally to a core-type or channel induction furnace, and more particularly, to such an apparatus equipped with an AC melt to bushing current detector for monitoring and indicating AC current from the bushing to ground for the purpose of determining if there is adequate and proper electrical isolation between the molten metal in the loop and the bushing.
2. Discussion of Related Art
Channel induction furnaces for melting, holding and pouring are common place in the foundry industry. Such devices are equipped with an inductor which heats the molten metal in the furnace by inductive means. The inductor may be of either a single or multi loop design. An inductor is essentially a transformer where the primary coil is located about a magnetic core which is surrounded by a secondary winding which consists of a short circuited molten metal loop. The molten metal in the loop is contained within a refractory lining that is confined by the bushing or cooling jacket and the inductor case, where an electrically insulated, metallic, often water cooled bushing is standard equipment.
As the molten metal in the loop is inductively heated, it gains temperature. Since the greatest amount of energy is induced on the inside of the loop with the energy falling off exponentially with radial outward depth, the hottest metal is on the inside of the loop. This poses a problem since the refractory has a porosity of 15-25%. This means that the refractory grains are surrounded by interconnected pores.
Molten metal from the loop flows into the interconnected pores in the refractory forming a saturation network. The depth to which the molten metal saturation takes place is determined by the thermal gradient in the refractory lining. At the point where the heat flow from the molten metal in the loop equals the heat being extracted by the bushing, the leading edge of the molten metal saturation network will solidify. This saturation process takes place within the first week of operation. For example, the metal saturation network in a 6 inch thick lining will solidify at about 2 inches from the bushing. This depth is considered a safe distance from the bushing.
Unfortunately, as the refractory lining between the loop and the bushing wears, the thermal gradient changes allowing the leading edge of the saturation network to move closer to the bushing. Consequently, at some point in time, the saturation network will come in contact with the bushing. When it does, the saturation network will melt a hole in the bushing allowing molten metal to run out through the bushing.
A molten metal run out through the bushing is defined as molten metal breaching it's containment by the refractory and bushing, allowing the head pressure of the bath in the upper case to force molten metal out through said breach. When a molten metal run out occurs, the result can be catastrophic with potential injury to personnel and/or major equipment destruction.
3. Disadvantages Prior Art
U.S. Pat. No. 6,208,682 to Archenholta et al. (March 2001) recognizes that a channel inductor is normally equipped with a cooling jacket (bushing) for cooling of both the housing and the coil, however fails to recognize other technical and practical aspects of the cooling jacket and chooses to eliminate same. Eliminating the bushing also eliminates the most effective means for forming the required refractory lining as well as the most effective means for setting up the required thermal gradient for shutting down the process of metal penetrating radially inward, too deeply into the refractory. Prior art makes use of various, expensive, consumable, difficult to install walls of metallic net or mesh backed with an electrically insulating material installed in the refractory. Electrically insulating materials tend to have some thermal insulating properties. Placing any kind of thermal insulating material in the refractory between the bushing and the molten metal loop will alter the thermal gradient and promote a greater depth of molten metal saturation into the refractory which is not desired. A requirement of the prior art detection means comprises a measurement means for monitoring the resistance in the refractory lining between the first and the second detection walls which requires an additional power source to generate the means of detection. Typically, no means of detecting and alerting for metal penetration and potential bushing run out is used, as the only known methods have been complicated, somewhat radical in concept, difficult to install, and expensive to constantly replace consumables with each campaign.